1. Field of the Invention
The present invention relates to the field of oil filters.
2. Prior Art
The preferred embodiments of the present invention are intended for use as drop-in engine oil filters, though it is to be understood that filters in accordance with the present invention may be used for other filtering functions such as the filtering of hydraulic fluids and the like. In the case of engine oil filtering, the oil filters themselves have typically been either of two general types. Previously the most common type of engine oil filter is the so-called spin-on filter, typically having a can-like structure having an internally threaded opening in the center of the top thereof, with a plurality of openings encircling the threaded opening and a face seal encircling the plurality of openings. When the spin-on filter is screwed onto a pipe on the filter mount, this seal will seal against an adjacent face of the filter mount, with one or more openings in the filter mount adjacent the threaded pipe providing oil communication to the plurality of openings. Thus oil flows through the plurality of openings, to the periphery of the paper filter element therein, through the filter element, and back out through the threaded pipe for distribution in the engine. These filters spin-on filters are convenient to use and typically easy to change, though have the disadvantage that the filter element enclosure, being part of the filter, is replaced every time the filter is changed.
Drop-in filters, on the other hand, are characterized by a filter enclosure which is a permanent part of the engine. As the name implies, the enclosure may be opened, the old filter removed therefrom, a new filter dropped in in its place, and the enclosure again closed. While this may be a bit messier operation than changing a spin-on filter, it has the economic advantage of reusing the enclosure itself; which should reduce the manufacturing cost of a drop-in filter to approaching that of the filter element within a spin-on filter. That cost savings, however, seems to be enjoyed more by the manufacturers of filters than by the consumer, as the retail prices of drop-in filters are not substantially different from the retail price of spin-on filters.
In the case of prior art drop-in filters, the pleated paper filter element provides a very substantial flow restriction for oil flow through the filter. Thus to provide adequate flow of oil in the engine, a bypass valve is used to provide an alternate oil flow path from the oil pump to the engine when the differential pressure across the oil filter reaches a predetermined level, typically approximately 2 psi. Thus most of the oil circulation through the engine is of oil which has bypassed the filter. Accordingly such filters are generally referred to as nominal extraction filters because of the relatively high bypass flow rate. No effort is generally made to seal the filter with respect to the enclosure to prevent oil flow from the enclosure inlet to the enclosure outlet by passing around the filter element. While the leakage of such oil can be substantial, it is of little consequence in such filters because of the fact that if oil leakage around the filter is avoided, the flow rate through the bypass valve would just increase accordingly. Thus the filtering provided by typical prior art drop-in filters is far from ideal.
In addition, prior art paper filters have been known to disintegrate in use, not only destroying the filtering capabilities, but further dispersing filter fragments throughout the engine, with the potential of clogging critical oil flow passages, causing greatly accelerated wear or even catastrophic failure of the engine. It is understood that this phenomenon has been traced to the use of synthetic oils, though in any event, it highlights the relatively fragile construction of such prior art filters and their potential for disintegration from known or unknown oils or additives.
A typical prior art drop-in filter is illustrated in FIGS. 1 and 2. As shown in these Figures, a pleated cylindrical paper filter element 20 is sandwiched between and cemented to top cap 22 and bottom cap 24. The bottom cap 24 is simply a flat disc-like cap with a hole in the center, top cap 22 being similar, though having protrusions 26 thereon for alignment and retention of the filter by the top of the filter enclosure on the engine, and further with a depression 28 leading to an opening in the center of the top cap 22. A sort of felt or fabric washer 30 is cemented to the bottom cap 24 to complete the assembly. Thus it will be noted that the sole support for the paper filter element is the paper filter element itself. There is also no outer protection for the paper filter element, which could be subject to mishandling and damage when being installed. Finally, course, used drop-in filters must be appropriately disposed of because of their classification as toxic waste. Thus there is an expense in properly disposing of a used filter. The specific prior art filter illustrated in FIGS. 1 and 2 is marked as covered by U.S. Pat. Nos. 6,086,763 and 6,554,139.